The art of lithographic printing relies on the immiscibility of oil and water, wherein the oily material or ink is preferentially retained by the image area of a lithographic printing plate. When a suitably prepared surface is moistened with water and an ink is then applied, the background or non-image area retains the water and repels the ink while the image area accepts the ink and repels the water. The ink on the image area is then transferred to the surface of a material upon which the image is to be reproduced; such as paper, cloth and the like. Commonly the ink is transferred to an intermediate material called the blanket which in turn transfers the ink to the surface of the material upon which the image is to reproduced.
A commonly used type of lithographic printing plate has a light-sensitive coating applied to an aluminum base support. The coating may respond to light by having the portion which is exposed become soluble so that it is removed in the developing process. Such a plate is referred to as positive-working. Conversely, when that portion of the coating which is exposed becomes hardened, the plate is referred to as negative-working. In both instances the image area remaining is ink-receptive or oleophilic and the non-image area or background is water-receptive or hydrophilic. The differentiation between image and non-image areas is made in the exposure process where a film is applied to the plate with a vacuum to insure good contact. The plate is then exposed to a light source, a portion of which is composed of UV radiation. In the instance where a positive plate is used, the area on the film that corresponds to the image on the plate is opaque so that no light will strike the plate, whereas the area on the film that corresponds to the non-image area is clear and permits the transmission of light to the coating which then comes more soluble and is removed. In the case of a negative plate, the converse is true. The area on the film corresponding to the image area is clear while the non-image area is opaque. The coating under the clear area of film is hardened by the action of light while the area not struck by light is removed. The light-hardened surface of a negative plate is therefore oleophilic and will accept ink while the non-image area which has had the coating removed through the action of a developer is desensitized and is therefore hydrophilic.
One of the more serious problems which can afflict negative-working lithographic printing plates is inability of the developer to remove all residual coating from the non-image areas of the plate. When sufficient residual coating remains, a condition exists for background sensitivity to occur during the printing process. Minimally, the effect would be to increase the amount of water required in the fountain solution. Under more severe conditions, ink may adhere to the background and ultimately to the printed sheet, thereby resulting in a condition known as "tinting" or "toning." Under extremely severe conditions, there is so much ink in the background that it is referred to as "scumming." Thus it is apparent that the lithographic printing process requires a background surface which will accept and hold water well, in order to provide effective rejection of the lithographic printing ink.
While many kinds of water accepting lithographic printing supports have been made, there is a continuing need for economically prepared water accepting supports.